1. Field of the Invention
The present invention relates to a process for the preparation of catalytically active multielement oxide materials which contain at least one of the elements Nb, W and the elements Mo, V and Cu, the molar fraction of the element Mo, based on the total amount of all elements of the catalytically active multielement oxide material, other than oxygen, being from 20 to 80 mol %, the molar ratio of Mo contained in the catalytically active mutielement oxide material to V, Mo/V contained in the catalytically active mutielement oxide material being from 15:1 to 1:1, the corresponding molar ratio Mo/Cu being from 30:1 to 1:3 and the corresponding molar ratio Mo/(total amount of W and Nb) being 80:1 to 1:4, in which an intimate dry blend also containing ammonium ions is prepared from starting compounds which contain the elemental constituents of the mutielement oxide material, other than oxygen, as components and said dry blend is thermally treated in an atmosphere having a low content of molecular oxygen at elevated temperatures, at least a portion of the ammonium ions contained in the intimate dry blend being decomposed at ≧160° C. with liberation of ammonia.
The present invention also relates to a process for the preparation of acrylic acid by heterogeneously catalyzed partial gas-phase oxidation of acrolein using catalysts which contain the abovementioned mutielement oxide materials as catalytically active materials.
The process, described at the outset, for the preparation of catalytically active mutielement oxide materials is known, as is the use of the multielement oxide materials obtainable thereby as active material in catalysts for the heterogeneously catalyzed partial gas-phase oxidation of acrolein to acrylic acid.
2. Discussion of the Background
DE-31 19 586 C2 discloses that a catalytically active mutielement oxide material which contains the elements Mo and V as base component can be prepared by preparing an intimate dry blend comprising ammonium ions from starting compounds which contain the elemental constituents or the multielement oxide materials as components, and thermally treating said dry blend at 380° C. in a gas stream which contains 1% by volume of molecular oxygen.
The resulting mutielement oxides are recommended as active material for catalysts for the catalytic partial gas-phase oxidation of acrolein to acrylic acid.
DATABASE WPI, Week 7512, Derwent Publication Ltd., London, UK; AN 75-20002 & JP-A 49097793 (Asahi Chemical Ind. Co.), Sep. 19, 1974, recommends the thermal treatment of corresponding intimate dry blends for the preparation of relevant mutielement oxide active materials with compete exclusion of molecular oxygen. EP-A 113 156 recommends carrying out the thermal treatment in an air stream. EP-A 724481 states that the thermal treatment should be carried out in such a way that the content of molecular oxygen at any point during the thermal treatment in the (gaseous) treatment atmosphere is from 0.5 to 4% by volume. In the exemplary embodiment, the molecular oxygen content in the thermal treatment atmosphere was 1.5% by volume.
In the exemplary embodiments of EP-A 714700, the thermal treatment of the intimate dry blend is carried out both in an air stream and in an atmosphere whose molecular oxygen content was 1.5% by volume.
DE-A 10046928, DE-A 19815281 and EP-A 668104 show that mutielement oxide active materials which are mentioned at the outset and have a multiphase structure are particularly suitable as active materials for catalysts for the heterogeneously catalyzed partial oxidation of acrolein to acrylic acid when at least one phase is separately preformed for the preparation of the intimate dry blend to be thermally treated and the thermal treatment is carried out in a gas atmosphere which continuously contains from 1.5 to 1.4% by volume of molecular oxygen.
A disadvantage of the prior art is that substantially all of it recommends a thermal treatment of the intimate dry blend at a content of molecular oxygen which is substantially constant over the duration of the thermal treatment in the associated gas atmosphere.
However, relevant mutielement oxide active materials obtained in this manner are not completely satisfactory with regard to activity and selectivity when used as active materials in catalysts for the heterogeneously catalyzed partial oxidation of acrolein to acrylic acid.